Many different polymers and materials have been added to polymer compositions in attempting to enhance the composition's impact strength or maintain the impact strength while enhancing other properties. For example, U.S. Pat. No. 5,118,753 (Hikasa et al.), incorporated herein by reference, discloses thermoplastic elastomer compositions said to have low hardness and excellent flexibility and mechanical properties consisting essentially of a mixture of an oil-extended olefinic copolymer rubber and an olefinic plastic. The olefinic plastic is polypropylene or a copolymer of polypropylene and an alpha-olefin of two or more carbon atoms. Modern Plastics Encyclopedia/89, mid October 1988 Issue, Volume 65, Number 11, pp. 110-117, the disclosure of which is incorporated herein by reference, also discusses the use of various thermoplastic elastomers (TPEs) useful for impact modification. These include: elastomeric alloys TPEs, engineering TPEs, olefinic TPEs (also known as thermoplastic olefins or TPOs), polyurethane TPEs and styrenic TPEs.
Thermoplastic olefins (TPOs) are often produced from blends of an elastomeric material such as ethylene based random copolymers, ethylene/propylene rubber (EPM) or ethylene/propylene diene monomer terpolymer (EPDM) and a more rigid material such as isotactic polypropylene. Other materials or components can be added into the formulation depending upon the application, including oil, fillers, and cross-linking agents. TPOs are often characterized by a balance of stiffness (modulus) and low temperature impact, good chemical resistance and broad use temperatures. Because of features such as these, TPOs are used in many applications, including automotive facia and wire and cable components, rigid packaging, molded articles, instrument panels, and the like.
Polypropylene (PP) homopolymers or PP random copolymers provide the desirable stiffness and clarity for many applications, but can suffer from poor impact properties due to a high Tg (0° C. for homopolymer PP, hPP). To overcome this deficiency, PP homopolymer is blended with PP copolymers and/or elastomers to improve its toughness, but often at the expense of its clarity and modulus.
Ideally the elastomer or compatibilizer should promote or produce elastomer particles that are small enough scale such that a minimum volume is needed to improve the impact performance without adversely affecting the modulus of the blend.
An additional improvement would be to develop an elastomer that improves the impact performance without adversely affecting its clarity or other preferred properties. Ideally, the modulus and clarity of the PP/propylene-containing elastomer blend product should be comparable to that of PP homopolymer.